Request backfill

Reprocess a Corpus's past Versions with a selected active ingestion plugin, to fill vector spaces of the generation the Corpus is routed to, typically a new evaluation space. The scope is the Corpus and, optionally, a window on when Quivr accepted its Versions. Only Versions segmented by the selected plugin whose segments all hold a vector in the served space and miss one in a target space are processed; live enrichment fills the target spaces for newer Versions once the backfill has started. A dry run is required. dry_run true answers 200 with the estimate and records it under the idempotency key. The same body with dry_run false then accepts the backfill as a queued Operation (202, Location). Without a dry run recorded under that key and scope, the answer is 409 dry_run_required. An estimated cost above the deployment's backfill.max_cost_without_confirmation needs confirm_cost true, otherwise 409 cost_confirmation_required. The same key with another scope is 409 idempotency_conflict, and an accepted key replays its Operation. A Corpus whose previous backfill has not finished is 409 backfill_in_progress. The backfill runs on its own task queue at the deployment's backfill.rate, pinned to the Pipeline Plan active when it starts. It can be paused, resumed and canceled, and resumes from its checkpoint after a restart. It never creates Record Versions or content events. Versions whose projected segments the plugin would cut differently are skipped and counted (segmentation_differs), and a rebuild re-segments them. Requires plugins:admin, on a key of the Corpus's Organization that grants the Corpus.